Method for using glass substrate surface modifiers in the fabrication of photochemically stable deep ultraviolet pellicles

ABSTRACT

A pellicle for use on a photomask reticle in conjunction with deep ultraviolet light wavelengths. The membrane of the pellicle comprises a purified fluoropolymer that has been spin coated on a glass substrate treated with a surface modifier and then separated and mounted on an aluminum frame. The aluminum frame has vents that filter air of contaminants and which allow an equalization of air pressure on both sides of the pellicle membrane when a peel-off backliner is in place on the opposite side of the frame. A permanent bond is made between the membrane and frame and a sticky adhesive is used to keep the backliner on the frame until peel-off. The sticky adhesive is such that the backliner may be re-attached a plurality of times.

COPENDING APPLICATIONS

This application is a division and a continuation-in-part of U.S. patentapplication Ser. No. 07/936,758, filed Aug. 21, 1992, and titled,PHOTOCHEMICALLY STABLE DEEP ULTRAVIOLET PELLICLES FOR EXCIMER LASERS,now U.S. Pat. No. 5,344,677.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to pellicles for protecting photomasksfrom particulate contamination and more particularly to pelliclessuitable for use with deep ultraviolet radiation as is present whenusing excimer lasers.

2. Description of the Prior Art

Pellicles are used as protective covers to keep particulate matteroutside of a focal plane of an optical apparatus so that a desirableimage is not disturbed. Pellicles generally comprise thin, transparentmembranes or films of polymer stretched over an aluminum frame that ismounted to form a hermetically sealed, dust-free enclosure over aphotomask reticle. Pellicles are widely used in semiconductormanufacturing of integrated circuit, both to protect photomasks fromparticulate contamination and to extend the mask life. Their majorfunction is to eliminate soft defects and improve die yield. The use ofpellicles by semiconductor manufacturers is reviewed in detail by RonIscoff in "Pellicles 1985: An Update," Semiconductor International(April 1985). Projection printing systems also use pellicles, as isdescribed by Shea, et al., in U.S. Pat. No. 4,131,363, issued Dec. 26,1978. A broad class of pellicles and a method for forming thesepellicles is also described by Winn, in U.S. Pat. Nos. 4,378,953 and4,536,240, issued on Apr. 5, 1983 and Aug. 20, 1985, respectively. Thefollowing U.S. Pat. Nos. also describe pellicles: 4,973,142, issued Nov.27, 1990, to Edward N. Squire, and assigned to du Pont; 4,948,851,issued Aug. 14, 1990, to Edward N. Squire, and assigned to du Pont;5,008,156, issued Apr. 1, 1991, to Gilbert H. Hong, and assigned toExion; and 4,657,805, issued Apr. 14, 1987, to Y. Fukumitsu, andassigned to Asahi Chemical.

Nitrocellulose has been widely used in pellicle manufacturing. Butnitrocellulose is not suitable for 248 nanometers (or shorterwavelengths) lithography because nitrocellulose is highly absorbing at248 nanometers and will rapidly degrade. The use of nitrocellulose hasalso declined because nitrocellulose is highly flammable and must bestored in a wetted condition. Nitrocellulose is also hygroscopic, whichmakes manufacturing under humid conditions difficult. Thus as a finishedproduct, pellicles of nitrocellulose wrinkle when wetted with water,which makes cleaning or storing under humid conditions a problem. Acritical problem with nitrocellulose pellicles is that thenitrocellulose material itself does not transmit ultraviolet (UV) lightwell enough for use in modern equipment that depend on the use of deepultraviolet light. Irradiation with ultraviolet light can also cause anitrocellulose pellicle membrane to become discolored, thus reducing itstransparency. Below two hundred and sixty nanometers, evennon-discolored nitrocellulose transmits less than seventy percent (70%)of incident light. This limitation in nitrocellulose, and also in MYLAR,when used in pellicles, is discussed by R. Hershel, in "PellicleProtection of IC Masks," A Report by Hershel Consulting, Inc. (August1981).

Advances in lithographic processes used in manufacturing integratedcircuits depend on reducing the wavelength of the incident ultravioletlight used in conjunction with pellicles. The development of a broadbandpellicle capable of transmitting ultraviolet light is described by I. E.Ward and D. L. Duly, in "Optical Microlithography III: Technology forthe Next Decade," SPIE, Vol. 470, pp. 147-154 (H. L. Stover, Editor),1984. Ward and Duly describe an antireflective layer that is coated onat least one side of a pellicle, in order to reduce any opticalinterference. Such optical interference is typically caused by internalreflections of light within a pellicle and is evidenced by anoscillating behavior in the transmission spectrum of a pellicle.Proposed solutions to this particular problem have included applyingantireflective coatings and controlling the thickness of the membrane.Antireflective coatings do not adhere well to a pellicle's surface. Theimperfect adhesion often then results in cracking and flaking of theantireflective coating, thus ruining the pellicle.

U.S. Pat. No. 4,657,805, issued Apr. 14, 1987, to Fukumitsu, et al.,discloses the use of thin fluoropolymer films to serve as antireflectivelayers for a pellicle. Multiple layers of the fluoropolymer films arecoated on a core layer pellicle to form a five-layer pellicle structurewith the indexes of refraction of the various layers being chosen toreduce internal reflection and scattering.

The ultraviolet transmitting pellicles of Ward are described morecompletely in a series of three patents. U.S. Pat. No. 4,482,591, issuedNov. 13, 1984, discloses a pellicle comprised of polyvinyl butyral resin(PBR) and the use of a ring with an adhesive side to remove the pelliclefrom a wafer. U.S. Pat. No. 4,499,231, issued Feb. 15, 1985, discloses apellicle comprising PBR and a dispersion of colloidal silica. U.S. Pat.No. 4,476,172, issued Oct. 9, 1984, discloses pellicles comprised of aPBR derivative that includes a silane moiety.

Problems also exist in the processes used to manufacture pellicles. Forexample, typically, a pellicle is formed by depositing a polymersolution on an inert substrate and then evaporating the solvent. Thisleaves the pellicle coated on the inert substrate. Removing the delicatepellicle from the substrate is a difficult, but a necessary step in theprocess. U.S. Pat. No. 4,536,240, issued to Winn, discloses a method foraccomplishing this task by bonding a frame to the pellicle and thenpeeling the pellicle off the substrate. In conjunction with thisprocedure, a suitable release agent can be applied to the substrateprior to applying the fluoropolymer solution and thus aid in removingthe pellicle. This procedure, however, results in a high number ofpellicles being ripped during the removal step.

Duly, et al., in U.S. Pat. No. 4,523,974, issued Jun. 18, 1985, disclosea method for manufacturing a pellicle from polymethylmethacrylate (PMMA)that includes the steps of applying a gold film to the surface of anoxidized wafer, coating a thin layer of PMMA on the gold film, removingthe PMMA and gold layers from the wafer and etching off the gold layer.

Microlithography trends for the last decade have been towards shorterand shorter wavelengths of ultraviolet radiation. The stepper radiationis changed from mercury G-line of 436 nanometers to I-line of 365nanometers. A state-of-the-art stepper utilizes krypton fluorideemission at 248 nanometers and XE-F at 194 nanometers to delineatefeature sizes around 0.3 micron.

Pellicles are well accepted by the photomask industry as an effectivemeans of protecting the cleanliness of masks used in microlithographicprocesses. When masks are pelliclelized and used in transferring imagesof IC design on mask to wafer, the pellicles serve not only as aprotective dust cover, but also as a part of the optics that do thelithographic imaging. Pellicle membranes must be photochemically stableto deep ultraviolet radiation, e.g., to wavelengths of 194 nanometersand 248 nanometers for excimer laser steppers. Pellicle membranes mustbe highly transparent into the deep ultraviolet range to guarantee highwafer throughput. Pellicle membranes must be very clean to ensure thatno defects result in the wafers being processed. Pellicle membranes mustbe able to attach to an aluminum frame with appropriate adhesives and bestrong, even at the typical thickness of 0.5 micrometers to 5.0micrometers, to ensure the assembled pellicles are stout enough forordinary use.

Although fluoropolymers have been described in the prior art as usefulfor deep ultraviolet pellicles with aluminum frames, a need neverthelessexists for a high yield method of casting the fluoropolymer films, apellicle for eliminating bursting due to trapped air when theatmospheric pressure changes and a backliner that cooperates withrobotics used in automated manufacturing facilities.

It is understood by those skilled in the art that pellicle manufacturinginvolves the coating of a glass substrate and a later peeling-off of aresulting membrane. Manufacturing yields are reduced by the tendency ofthe membrane to adhere too strongly to the glass substrate. Inparticular, this is a problem in the manufacture of deep ultraviolet(DUV) pellicles made of fluoropolymers.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to provide a methodof casting the fluoropolymer films.

It is a further object of the present invention to provide a pellicleframe that vents differences in atmospheric pressure to eliminatebursting in the pellicle membrane.

It is another object of the present invention to provide a pellicle witha backliner that is compatible with robotics used in automatedmanufacturing facilities.

Briefly, an embodiment of the present invention is a pellicle with afluoropolymer membrane that has been spin coated on a nitrocellulosesubstrate before separating, a vented aluminum frame to which theseparated fluoropolymer has been permanently bonded, and a stiffpeel-off backliner that is secured to the aluminum frame with a stickyadhesive that permits multiple cycles of detachment and reattachment ofthe frame to backliner.

An advantage of the present invention is that it provides a pelliclethat is economical to manufacture because very few of the fluoropolymermembranes are damaged during separation from their substrates.

Another advantage of the present invention is that it provides apellicle that will not burst in or out due to changes in atmosphericpressure.

A further advantage of the present invention is that it provides apellicle that is compatible with robotics used in automatedmanufacturing facilities.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodimentswhich are illustrated in the various drawing figures.

IN THE DRAWINGS

FIG. 1 is a perspective exploded assembly diagram of a pellicle andshipping container of the present invention;

FIG. 2A is a perspective cutaway diagram of the vent in a part of theframe of the pellicle of FIG. 1;

FIG. 2B is atop elevation view of the vent of FIG. 2A;

FIG. 2C is a side elevation view of the vent of FIG. 2A; and

FIG. 3 is a cross-sectional diagram of the fluoropolymer membrane of thepellicle of FIG. 1 before being separated from the nitrocellulose onglass substrates on which it is formed by spin coating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an embodiment of a pellicle of the present invention,referred to by the general reference numeral 10. The pellicle 10comprises an amorphous fluoropolymer membrane 12, an aluminum frame 14including vents 16 and a backliner 18 with a thumb tab 20 and anadhesive sticker 21. Acceptable fluoropolymer materials useful formembrane 12 include tetrafluoroethylene (TFE)-hexafluoropropylene (HFP)copolymer, vinylidene fluoride homopolymer, vinylidene fluoride(VdF)-TFE copolymer, TFE-perfluoroalkyl vinyl ether copolymer andVdF-ethylene copolymer. Preferred fluoropolymers are dissolvable insolvents and formable into uniform films by the spinner coating method.Pellicle 10 is intended for use in applications with 193 nanometerseximer laser radiation. The materials mentioned herein for membrane 12are suitable for such use. Numerous application environments compriseultraviolet radiation at 248 nanometers, and the materials mentionedherein for membrane 12 are also suitable for such use. Since suchradiation may degrade membrane 12, the preferred thicknesses formembrane 12 ranges from a low of 0.5 microns to a high of three microns.

Du Pont has developed at least two fluoropolymer materials pertinent topellicle manufacture and has made them commercially available. Forexample, see U.S. Pat. No. 4,973,142, issued Nov. 27, 1990, and U.S.Pat. No. 4,948,851, issued Aug. 14, 1990, both to Edward N. Squire. Afirst amorphous fluoropolymer is marketed by Du Pont as "AF-2400", whichis a copolymer of perfluoro-2,2-dimethyl-1,3-dioxide (PDD) andtetrafluoroethylene (TFE), with 85% PDD and 15% TFE. The glasstransition temperature of this copolymer is 240° C. A second amorphousfluoropolymer is sold as "AF-1600", which is a copolymer of PDD and TFEwith 65% PDD and 35% TFE. The glass transition temperature of thislatter copolymer is 160° C. Generally, the higher the concentration ofPDD, the higher will be the rigidity and the glass transitiontemperature. Both of these copolymers are soluble in 3M Company FCFLUORINERT liquids (e.g., FC 72 which has a boiling point of 56° C., andFC 40 which has a boiling point of 155° C.). AF-1600 can be prepared ina mixture of FC-72: FC/40 (1:3) to a concentration of 8-10% and AF-2400to 2-3%. AF-1600 has exhibited adequate solubility to form acceptablepellicle membranes. Ashahi Glass (japan) markets aper-fluoro-cyclo-oxy-aliphatic-homopolymer under the name "CYTOP", whichhas a glass transition temperature of 108° C. and is more soluble inper-fluorited liquids. However the films it forms are not as rigid asthe ones made of AF-1600 and AF-2400. Nevertheless, AF-1600 and CYTOPare suitable for application environments comprising ultravioletradiation in the range of 190-500 nanometers.

For more information regarding such fluoropolymers, refer to U.S. Pat.No. 4,657,805, issued Apr. 14, 1987, to Fukimitsu, et al. For moreinformation regarding spin-coating, refer to U.S. Pat. No. 4,536,240,issued Aug. 20, 1985, to R. Winn.

Frame 14 is typically 3.75 inches by 4.75 inches. The perimeter materialof frame 14 is approximately 0.25 inches by 0.0625 inches. Vents 16allow air but not contamination to pass through frame 14 such thatchanges in atmospheric pressure will not tend to burst membrane 12 in orout when both membrane 12 and backliner 18 are sealed to frame 14 orwhen pellicle 10 is in use on a photomask reticle. A high-strength,ultraviolet-curable cement is preferably used to permanently sealmembrane 12 to frame 14. A tacky, restickable type adhesive is used toform a temporary seal between frame 14 and backliner 18 to allowmultiple cycles of peel-off and reattachment of backliner 18 to frame14. (Such an adhesive is familiar to lay persons as that used in theyellow 3M Company POST-IT note pads.) Sticker 21 is color-codeddifferently on each side, e.g., one side gold and the other side silverin color. Such color-coding assists a user in consistently returning thesame side of backliner 18 to frame 14.

Pellicle 10 is preferably transported inside a shipping container bottom22 which includes a pair of finger notches 24 and a pair of thumb tabnotches 26. Pellicle 10 rests on a ledge 28 when deposited withinshipping container bottom 22. A clear, see-through plastic shippingcontainer cover 30 fits over shipping container bottom 22 such thatpellicle 10 may be seen by a user and protected within during transport.The shipping container bottom 22 and cover 30 are such that they arecompatible with robotic manipulators that can open cover 30, extractpellicle 10 and separate backliner 18 from frame 14. Backliner 18 isalso stiff enough to allow a robotic vacuum lifter to lift awaybackliner 18 from the adhesive on frame 14. Such stiffness permits anautomated system to remove backliner 18 without folding, wrinkling orstretching backliner 18. Pellicle 10 is then attached by roboticmanipulation to a photomask reticle. Backliner 18 guards the air volumewithin frame 14 and behind membrane 12 from contamination until finalmounting on the photomask, which is preferably done in a cleanroomenvironment. This function is especially important because it is thearea between the photomask and membrane 12 that will be in the focalplane of the optics associated with the photomask. Any dirt orcontamination within the focal plane will create optic anomalies thatcause wafer processing defects. Any dirt or contamination outsidepellicle 10, and therefore outside the focal plane, cannot be focusedsharp enough to cause such defects.

FIGS. 2A, 2B and 2C illustrate the construction of vent 16 whichcomprises a pair of holes 40 and 42 and a channel 44 within sidewall 14.Preferably, hole 40 and hole 42 are placed near opposite ends of thechannel 44 such that air flow through vent 16 must pass through thelength of channel 44. Holes 40 and 42 are therefore not straight throughthe wall 14, but rather are offset with each hole leading into thechannel 44, one from inside the pellicle cavity and the other from theoutside environment. Holes 40 and 42 are typically of 0.2-0.6millimeters inside diameter and channel 44 has an inside width of 0.7millimeters. The length and depth of channel 44 may be tailored to fit afilter material 46 (FIGS. 2B and 2C only) that may alternatively bedisposed within channel 44. The inside geometry of channel 44 may bemodified to gain certain benefits. For example, increasing the length ofchannel 44 may enhance the particle trapping capability, particularly ifthe adhesive used between frame 14 and backliner 18 forms one boundaryof channel 44. Particles may also be blocked by shaping channel 44 in azig-zag fashion. The air velocity of particles within channel 44 mayalso be decreased by shaping channel 44 like a horn. Tests by theinventor indicate that pellicle 10 when placed in the cargo bay of ahigh altitude commercial airliner will be able to bleed-off pressuredifferentials through vent 16 fast enough to cope with the normal rateof climb and descent of such aircraft before membrane 12 will beinjured. The material selected for filter 46 must support high flowratesat the necessary pore sizes. Good tensile strength and resistance tofracture under lateral stresses is also important. Filter 46 maycomprise a 3.0 micrometer filter of polytetrafluoroethylene, ahydrophobic material immune to wetting by the absorption of moisture.Fluoropore Company makes such filter material and sells it commercially.Filter 46 is attached to frame 14 with an adhesive, e.g., 3M Companyadhesive 4952, such that the walls are at least one millimeter thick.The adhesive should be allowed to cure for at least thirty minutes afteruse before pellicle 10 is washed.

Pellicle membranes 12 are fabricated on a reusable nitrocellulosesubstrate according to the steps listed in Table I.

                  TABLE I                                                         ______________________________________                                        STEP     DESCRIPTION                                                          ______________________________________                                        1        Depositing a film of nitrocellulose on a                                      super-polished photomask-grade soda lime                                      glass substrate. Both the nitrocellulose and                                  glass substrate must be substantially free of                                 any defects or contamination. The                                             nitrocellulose deposited in a film that is                                    approximately 0.5 to 3.0 micrometers thick,                                   and should present a clean, non-stick                                         surface.                                                             2        Depositing a film of fluoropolymer, such as                                   CYTOP or amorphous fluoropolymers 1600 or                                     2400, to a thickness of approximately 0.5 to                                  3.0 micrometers over the nitrocellulose film.                                 A well-tuned spinner may be used to obtain a                                  such a film thickness.                                               3        Bonding perimeter edges of the fluoropolymer                                  film to pieces of mending tape with a fluoro-                                 material, e.g., KEL-F-800.                                           4        Attaching with the mending tape a stainless                                   steel frame on top of the stack of glass,                                     nitrocellulose, fluoropolymer, and fluoro-                                    adhesive films.                                                      5        Peeling off the combination of film, mending                                  tape and stainless steel frame.                                      6        Attaching only the fluoropolymer film of the                                  combination of film, mending tape and                                         stainless steel frame to an aluminum frame.                          7        Trimming away the stainless steel frame and                                   mending tape from outside the perimeter of                                    the aluminum frame such that a taut                                           fluoropolymer film is stretched over the                                      aluminum frame.                                                      ______________________________________                                    

The first four steps of the process described in Table I results in thestructure illustrated in FIG. 3 which comprises a soda lime glasssubstrate 50, a nitrocellulose intermediate layer 52, a fluoropolymerlayer 54, a fluoro-adhesive layer 56, a mending tape layer 58 and astainless steel frame 60. After a lifting-off with mending tape layer58, layers 54, 56, 58 and frame 60 stay together and separate from layer52, which remains on the glass substrate 50 and is typically left in acondition good enough to be reused. The prior art methods differ fromthe present invention in at least two ways. First, in the presentinvention, glass substrate 50 is coated with the nitrocellulose layer 52before the additional layer of fluoropolymer 54 is applied. This takesadvantage of the fact that the fluoropolymer layer 54 separates rathereasily at the nitrocellulose-to-fluoropolymer interface. Iffluoropolymer layer 54 were put directly on to glass 50, theglass-to-fluoropolymer interface would be very difficult to separate,and the film would probably tear in the attempt. The prior art solvedsuch problems with the use of various solvents. Second, fluoro-adhesive56 is applied to the fluoropolymer 54 such that the film can be liftedusing a combination of mending tape 58 and stainless steel frame 60. Themending tape may be SCOTCH™ brand Magic Tape™ 810, as sold by 3MCommercial Office Supply Division (St. Paul, Minn. 55144-1000).Fluoropolymer 54 is then transferred to an aluminum frame, such as frame14 (FIG. 1) and becomes what is identified as membrane 12 in FIG. 1after trimming away stainless steel frame 60.

The method of fabricating pellicle membrane 12, described herein, isrelated to that described in U.S. Pat. No. 5,008,156, issued Apr. 16,1991, to Gilbert H. Hong, the present inventor, and which patentdisclosure is incorporated herein by reference. In general, some priorart methods start by preparing a coating solution of polymer in asolvent. This solution is filtered through a filter with a pore size of0.2 micron, or smaller, in order to remove contaminants, and thusimprove the clarity of the finished film. A class one hundred (orbetter) clean room is used during the pellicle manufacture to avoidairborne contamination. After filtering, the fluoropolymer solution isapplied to a super-flat, smooth and defect-free substrate, usingconventional spin-coating techniques. (Spin-coating is widely used inthe semiconductor industry for obtaining polymer films and most of theaccepted industry practices for obtaining defect-free coatings can beadapted to pellicle manufacturing.) Typically, a speed of about onethousand RPM is used to obtain a high quality two to three micronpellicle. Substrates of soda-lime glass are preferred. Prior to use, theglass substrate is cleaned by scrubbing in deionized water withdetergent. The substrate is then rinsed with copious amounts ofdeionized water. This is followed by a second rinse with isopropylalcohol using ultrasonic agitation. The substrate is dried in adegreaser tank with a chlorinated fluorocarbon, e.g., FREON. Inconsideration of the environment, other types of chemical degreasers maybe preferred. After spin-coating, any residual solvent is removed frompellicle membrane by heating in a super-clean oven for about thirtyminutes at approximately 60° to 90° C. This heating serves to increasemembrane tensile strength by reducing stresses in the fluoropolymer.Additional layers may be added through additional spin-coating steps. Incertain applications, and after drying, pellicle membrane is peeled-offof the glass substrate prior to being mounted to a square frame. Thisimproves the yield of pellicle membranes by reducing membrane breakage.Typically, the square frame is made of stainless steel and has athickness of about twenty mils and an inside dimension of six to seveninches, depending on the size of pellicle membrane. In general, thesquare frame is attached to pellicle membrane by adhesive strips, suchas 3M SCOTCH brand Magic Tape. A membrane assembly, with the glasssubstrate on one side and the square frame on the other, is submergedinto or sprayed with deionized water for about five minutes. An adhesionfailure at the interface of the pellicle membrane and glass is inducedby the parting action of the deionized water. The pellicle membraneseparates from the glass substrate and yet remains attached to squareframe. Either gentle heating or ambient evaporation heating will removewater droplets from the surface of the pellicle membrane. After thepeel-off and drying, the pellicle membrane can be transferred to aframe. This is done by placing the square frame containing the pelliclemembrane on top of the frame to which a permanent adhesive haspreviously been applied. After the permanent adhesive has hardened, thepellicle membrane is separated from the square frame by trimming awayexcess membrane material along the frame to yield the edge-mountedpellicle membrane. In typical applications, the thickness of pelliclemembrane is chosen to be either 0.85 microns or 2.83 microns. In somevariations, the thickness of pellicle membrane is varied to reduceoptical interference effects or to adjust the strength of the membrane.

Fluoropolymers can generally be dissolved in a perfluorinated fluid,such as FLURINERT by 3M Company. However, the fluoro-adhesivespreferably comprise material that is soluble in traditional solvents,e.g., methyl-ethyl ketone, ethoxy-ethyl acetate or propylene-methylacetate. Good results have been obtained with KEL-F 800 (3M Company),and KYNAR 7201 and 9301 by Pennwalt have been proven to give acceptableresults. The second layer must be dissolvable by a solvent that will notattack the first previously-spun layer.

Multiple-antireflective coated deep ultraviolet pellicle membranes 12can be made by using a structure comprising a first fluoropolymer layer,a second fluoroadhesive layer with high-index, e.g., 1.435, and a thirdfluoropolymer layer. For example, the first layer comprises AF-1600, thesecond layer comprises KEL-F, and the third layer comprises AF-1600.

Two types of experiments were performed by the inventor. A relaxationexperiment involved swelling of the membrane a distance dx, and thenmeasuring the time (dt_(return)) required for dx to approach zero. Aconstant dx, or venting experiment involved pressurizing pelliclemembrane 12 until it swelled to a given dx, measuring the differentialpressure (dP), and then timing how long (dt_(vent)) the membrane 12 canbe held at dx by controlling the rate of pressure drop through a vacuumvalve. In this experiment the pellicle membrane 12 cavities are oftennearly completely evacuated. The experiments were conducted in anon-cleanroom environment, in order to simulate a worst case scenario interms of membrane efficiency and possible loading under severeconditions. The relaxation data provides a rough estimate of the averageflowrate from a filtered pellicle membrane 12 whose vent hole is sorestricted that venting times would be prohibitively long to measure.The venting data allows one to calculate the average flowrate throughthe vent hole at a given constant pressure as well as the rate ofpressure drop needed to maintain this equilibrium. Four differentpellicle membranes 12 have been used: an Exion Technology, Inc. (SanJose, Calif.) NI-108-63-B-G with a three millimeter diameter hole forvent 16, an Exion Technology PE-107-31-A-T with a groove and four 0.5millimeters diameter holes for vent 16, an Exion TechnologyPB-107-31-A-T without a groove and with four holes of 1.0 millimeters,0.7 millimeters, 0.7 millimeters, and 0.2 millimeters diameters forvents 16, an Exion Technology PE-107-31-A-T with two recessed holes withan inner diameter of 0.6 millimeters for vent 16, and an ExionTechnology CA-122V-40-B-T.

Three types of adhesives have been used: 3M Company 447 (ten milrubber-based), Norwood KC8031 (thirty mil), and 3M Company 4952 (45 mil,acrylic-based). The latter two adhesives were cut into rectangularshapes with holes for vents 16 of approximately 0.07"×0.018", with aminimum side wall thickness of 0.7 millimeters. This will increase theusable filter area, since the filter membrane may bottleneck the airflow.

Five different Millipore membrane filters have been used for filter 46(FIGS. 2B and 2C): a 0.2 micrometer pore size Fluoropore (PTFE with ahigh density polyethylene backing), a 0.5 micrometer Fluoropore, a 1.0micrometer Fluoropore, a 3.0 micrometer Fluoropore (PTFE with apolypropylene backing), and a 8.0 micrometer MF-Millipore (mixed estersof cellulose). All of the membranes were cut to the appropriate sizefrom larger discs.

Pellicle 10 preferably can withstand stowage while being transported inthe non-pressurized cargo hold of a commercial airplane climbing 10000feet in three minutes. This creates an average rate of air pressure dropof 0.058 inches of mercury per second (" Hg/s). For pellicles 10, a dPof less than 1" Hg has been observed, which will cause less than 0.5millimeters of deflection of membrane 12. The potential pressure droprate could be as high as 0.067" Hg/s, however most cargo planes are atleast partially pressurized and the probable rates are much less.

For vent holes 40 and 42 there may be no real optimal hole size, sincemicron pore size membrane filters restrict air flow by several percentof the uninhibited flow. The hole size of holes 40 and 42 should be aslarge as possible, leaving approximately at least one millimeter ofspace on frame 14 around holes 40 and 42 for adhesive. Alternatively,the size of holes 40 and 42 can be kept relatively small while thenumber of holes can be increased in compensation.

Of the many membrane filters that Millipore manufactures, only theMF-Millipore and the Fluoropore series were found to have met thenecessary requirements for pore size and high flowrate. The MF-Milliporefilters do not appear to be suitable for this application because theyhave insufficient tensile strength and fracture under very small lateralstresses. The Fluoropore filters, on the other hand, can stretch withouttearing. In addition, they are made of polytetrafluoroethylene, ahydrophobic material, and are therefore immune to wetting by theabsorption of moisture. Depending on the pore size, wetted membranes canrequire from one to twenty pounds per square inch (PSI) of pressuredifference to clear. Of the three Fluoropore filters tested, the 3.0micrometer filter performed the best, in terms of realizable flowrate.Actual production using this filter may, however, pose several problems,e.g., the polypropylene support side of the filter is fibrous and shedswhen abraded, the stiff PTFE surface adheres only weakly to both theNorwood and 3M Company 4952 adhesives, and the filter 46 may peel offunder stress. The adhesion is better with the other three Fluoroporefilters because of their malleability. The PTFE side of the 3.0micrometer filter also appears to be more fibrous than the others,resulting in a tendency to be sometimes pulled apart under stress. The0.2 micrometer, 0.5 micrometer, and 1.0 micrometer Fluoropore filtersare all supported by a web-like HDPE backing. In most cases, thisbacking will not affect the flowrate, but for small vent holes (0.1millimeter radius) this material should and can be carefully peeled offof the PTFE membrane.

Adhesive configurations have consisted of a six millimeters by fivemillimeter piece of 3M Company 447 with a hole nearly the same size asvent holes 40 and 42. Other tests changed to Norwood and 3M Company 4952with a larger, rectangular hole because the larger available filtersurface area partially un-bottlenecked the flow and the greaterthickness allowed the flow out of the orifice to more fully develop,resulting in better usage of the available surface area. There are anumber of difficulties which must be resolved in manufacturing. Aprincipal concern is the precision cutting of holes 40 and 42. For apellicle 10 with a 3.1 millimeter standoff, for example, the adhesive is3.1 millimeters wide while holes 40 and 42 are 1.7 millimeters ID. Thispermits walls of 0.7 millimeters on either side. With Norwood thirty mil(0.76 millimeters), the height of the walls is roughly equal to theirwidth. For 3M Company 4952, the walls are 0.4 millimeters higher thanthey are thick. Both are spongy foam adhesives, and the result is anextremely difficult material to cut cleanly and precisely. After theadhesive is cut, then there may be additional problems with theapplication of filter 46 precisely to the adhesive and then the adhesiveto frame 14. Other problems include possible outgassing of 4952, thedurability of the filter-to-adhesive bond over time and thecompatibility of a raised filter element with existing pellicle handlingand mounting fixtures.

Nevertheless, it appears that the best solution at present in terms offlow is to include in vent 16 two to four large sized holes (0.4millimeters to 0.5 millimeters) combined with the 3.0 micrometerFluoropore filter for filter 46 attached to frame 14 with 3M Company4952 adhesive cut such that the walls are at least one millimeter thick.Since the bond strength of 3M Company 4952 is supposed to build up in alogarithmic fashion over a 72 hour period, the completed assembly ofpellicle 10 allowed to sit undisturbed for at least a half hour beforewashing. Possible modifications and configurations:

In an alternative embodiment, vent 16 is replaced by a recessed hole. Asmall circular filter is attached inside using either double-sticky tapeor epoxy glue such that the filter material is flush with or below thesurface of frame 14. This may resolve a potential problem ofcompatibility with existing pellicle fixtures, as well as relieving anyconcerns regarding the accidental removal of filter 46 in vent 16 sincethe recessed filter cannot be brushed out once it is attached inside thedepression.

The present invention includes process schemes that use an adhesionpromotor, a fluoropolymer, a surface modifier and a glass substrate. Thesurface modifier acts as a release agent to facilitate the separation ofthe fluoropolymer membrane that has filmed on the glass substrate. Theadhesion promoter is used to enhance the adhesion of the film membraneto the frame.

Surface modifiers are, in general, regarded as release agents. However,in context with the present invention, such surface modifiers aredefined as those having any affect on the surface. The inventor'sresearch has demonstrated that it is sometimes necessary to promoteadhesion so that additional layers of material may be deposited.Therefore, a surface modifier preferably optimizes the adhesion foradditional deposited films, but not so strongly that yields are severelycomprised by problems in peeling-off the whole film.

Surface modification research conducted by the present inventor withdeposits of thin organic films with surface modifiers such asnitrocellulose has led to the following conclusions. The glass surfacesof the substrate should be modified by depositing a suitable surfacematerial that expresses an affinity for glass that exceeds its affinityfor fluoro-film. The resulting structure allows the fluoro-film to beseparated from the surface modifier which stays with the glass substrateand can therefore e reused The adhesion of the fluoro-films with thesurface modifier must be optimized to produce an affinity just greatenough to support spin coating. Otherwise, a coating of fluoro-filmwould be prevented. However, that affinity cannot be so great as tocompromise the later peeling-off of the film. The surface modifier mustalso be substantially pure. Filtering the surface modifier helps to formnear perfect thin films with an optical smoothness that is as good asthat of the surface of the glass substrate.

In particular, the research of the present inventor has led to the useof nitrocellulose, cellulose acetate, ethyl cellulose, cellulose acetatebutyrate, polyvinyl butyral (PVB) and silicones, as surface modifiers.Acceptable silicones include those manufactured in the United States byGeneral Electric (GE) as UV9300, UV9310C, the SL6000 series and theSL5000 series. Curing is done with heating or ultraviolet light, asdirected by the manufacturer.

Therefore, with reference to FIG. 3, the nitrocellulose intermediatelayer 52 can be substituted by materials which include nitrocellulose,cellulose acetate, ethyl cellulose, cellulose acetate butyrate,polyvinyl butyral (PVB) and silicones.

Fluoropolymer 1(AD) and 2(AS) appear to work the best with theultraviolet-cured type of silicones. Fluoropolymer 2(AS) appears toworks well with the PVB. In general, many pairs of film material andsurface modifiers are possible.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that thedisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:
 1. A method for forming a fluoropolymer film on areusable nitrocellulose substrate and then separating the fluoropolymerfilm, the method comprising the steps of:coating a nitrocellulosesubstrate on a super-polished photomask-grade soda lime glass substratesuch that both said nitrocellulose substrate and said glass substrateare free of any defects or contamination, said nitrocellulose substratebeing a film 0.5 to 3.0 micrometers thick and presenting a clean,non-sticking surface to fluoropolymers; spin coating a fluoropolymerfilm on top of the nitrocellulose films to a thickness of approximately0.5 to 3.0 micrometers; bonding mending tape to said fluoropolymer filmwith a fluoroadhesive; attaching a stainless steel frame placed on topof said glass-nitrocellulose-fluoropolymer-fluoroadhesive combinationwith said mending tape; and peeling off said fluoropolymer film fromsaid nitrocellulose substrate such that said fluoropolymer film remainsattached to said stainless steel frame.
 2. A method for forming afluoropolymer film on a reusable substrate and then separating thefluoropolymer film, the method comprising the steps of:coating asubstrate selected from the group of cellulose, acetate, ethylcellulose, cellulose acetate butyrate, polyvinyl butyral and siliconeson a superpolished photomask-grade soda lime glass substrate such thatboth said substrate and said glass substrate are free of any defects orcontamination, said substrate being a film 0.5 to 3.0 micrometers thickand presenting a clean, non-sticking surface to fluoropolymers; spincoating a fluoropolymer film on top of said substrate film to athickness of approximately 0.5 to 3.0 micrometers; bonding mending tapeto said fluoropolymer film with a fluoroadhesive; attaching a stainlesssteel frame placed on top of said glass substratefluoropolymer-fluoroadhesive combination with said mending tape; andpeeling off said fluoropolymer film from said substrate such that saidfluoropolymer film remains attached to said stainless steel frame.